Problem partitioning when installing snow leopard with tiger

I have been trying to instal Snow Leopard 10.6.3 on a Mac Pro running Tiger. Apparently it should be straight forward as one should be able to partition the hard drive to create a new partition for the installation of SL but whenever I try to do this it fails.I am told to make the partition smaller for partitioning to work but whatever size I make a partition I get the same response.Basically the partition process appears to be useless as any newly sized partition isn't small enough.

I would dearly love to make this work because although I have an empty external hard drive which I could use to copy and store my Tiger files on, Tiger still has 120 gigs including all my applications.That assorted Tiger "stuff" will not fit on the new drive unless I prune it severely and I am not confident trying to prune items I do not even understand.
I'm sure there must be something I have overlooking that would enable me to proceed with this partitioning process.It sounds straight forward, it looks straight forward but it just isn't working. I would appreciate it if someone could offer some advice on how to do this successfully.
Thanks
Limey